Cryogenic air separation systems routinely utilize what is often referred to as liquid pumping for product pressurization. Liquid pumping refers to a direct mechanical compression of a cryogenic liquid product followed by vaporization against a warm condensing fluid. In this process, the refrigeration contained in the pumped liquefied product is imparted through indirect heat exchange to the compensating/condensing fluid. Such an approach is particularly useful for purposes of specialized product pressurization. In particular, the expense of oxygen compressors and related safety issues can be avoided through liquid oxygen pumping. There has been increased interest in processes employing full liquid pumping. In such processes oxygen is liquid pumped directly to the sendout (pipeline) pressure and vaporized within the process. The advantage of such processes stems from the complete elimination of the oxygen compressor. The complications associated with full oxygen pumping stem from the very high pressure air streams required for liquefaction. These high pressure air streams create a thermodynamic mismatch within the primary heat exchanger and hence added power consumption.
In many instances air is the preferred compensating fluid for vaporizing pumped liquid oxygen. A complication associated with full oxygen liquid pumping stems from the fact that air pressures in excess of the critical point, 547 pounds per square inch absolute (psia), are often required to vaporize the liquid oxygen. At oxygen pressures below the oxygen critical point (737 psia) substantial heat exchange inefficiencies are incurred. As a consequence, there exists substantial room for improvement in terms of heat exchange design approach. Moreover, it has been found that liquid pumped oxygen processes are not typically amenable to variable liquid production.